1. Field of the Invention
The present invention relates to a building board which is suited for use for constituting the external wall of building, in particular, building board whose back surface is improved in waterproof property. The present invention also relates to a manufacturing apparatus for such a building board and to a prefoamed plastics.
2. Description of the Related Arts
FIG. 1 illustrates a flow chart for explaining a continuous coating process of a building board according to the prior art. Namely, this continuous coating process involves the steps of: introducing, as a raw substrate, a cement substrate at first into the coating line (step S1); forming a back-coating by the coating of a back sealer on the rear surface of the cement substrate by means of a roll coater (step S2); drying that (1) (step S3); forming a under coating by the spraying of a sealer all over the top surface of the cement substrate (step S4); drying that (2) (step S5); forming an intermediate coating by the spraying of a color paint whose color is the same as that of the concave portions all over the top surface of the substrate (step S6); drying that (3) (step S7); forming a top coating by the coating of a paint on the concave portions of the substrate (step S8); drying that (4) (step S9); applying a decorative coating, if desired, by means of sputtering for instance (step S10); performing a clear coating by the spraying of a clear point all over the top surface of the substrate(step S11); drying that (5) (step S12); cooling that coated layers by air cooling (step S13); performing the inspection of the finished building board (step S14); packing the finished building board (step S15); and shipping the package, thus completing the flow.
As shown in FIG. 1, in order to provide the rear surface of the substrate or building board with a waterproofness of some degree, a back sealer coating is performed in the beginning of the coating process. The coating weight in this case is about 45-55 g/m2 in the wet state, and the thickness of the sealer after drying is about 10-20 xcexcm. Since the sealer is applied to the porous back surface of the cement substrate, the thickness of the sealer is ultimately thinned by the amount that has been impregnated into the porous interior of the substrate.
The term xe2x80x9csealerxe2x80x9d herein means a paint to be employed for the purpose of filling the pores existing on a porous and absorptive coating surface with the paint. For this purpose, a colorless and transparent acrylic urethane-based paint is extensively employed.
The back surface of cement substrate (i.e. a board not coated with a paint) that can be manufactured in the ordinary manufacturing line is approximately flat, but is actually accompanied with minute projected and recessed portions as well as a large number of voids as a whole, so that the density of board in the thickness-wise direction thereof is not necessarily uniform.
Therefore, it is difficult, in the back sealer-coating step of the continuous coating process of building boards, to avoid the generation of partial non-uniformity of coating on a sealer-permeable surface. If such a non-uniformity of coating is generated, it becomes impossible to expect a sufficient waterproofness as desired of the building board.
However, it is not necessarily appropriate, for the reason of inviting a substantial increase in manufacturing cost, to increase the quantity of the sealer to be applied to the back surface of the building board which portion is usually hidden from one""s eyes.
The coating of sealer has been conventionally performed with a view that, since the sealer coated on back surface is not directly exposed to rain fall in contrast to the coated layer formed on the front surface of building board, it would be sufficient as the function of the sealer layer if it can provide the back surface of substrate with some degree of waterproofness. Therefore, a little attention has been paid on other physical properties such as scratch resistance, and hence, the layer of sealer that has been conventionally applied to the back surface of substrate is not necessarily sufficient enough in terms of mechanical strength.
Due to such a conventional thinking, there has been much possibility that scratching may be accidentally generated in the layer of sealer during the transportation or attaching operation of the building boards. If scratching is happened to be generated in the layer of sealer, it is no more possible to prevent the permeation of water through such a scratched portion. There is also a possibility that a fatal defect such as the breakage of rabbet portion of building board may be generated due to a shock given to the building board.
The present invention has been accomplished under the aforementioned circumstances, and therefore an object of the present invention is to provide a building board which can be easily and reliably manufactured without necessitating the step of coating a back sealer and a drying step to be followed, and which is capable of effectively exhibiting an excellent waterproofness which has been conventionally aimed at by the layer of sealer formed on the back surface of building board. Other objects of the present invention are to provide a manufacturing apparatus for such a building board and to a prefoamed plastics to be employed for the manufacture of such a building board.
Namely, the building board proposed by the present invention is featured in that a ground of cement substrate is backed with a plastic sheet.
When the plastic sheet is printed with a pattern, it is possible to provide the back surface of building board with a sort of design, thus making it useful in the manufacture of a simplified facing.
When the plastic sheet is formed of a polyethylene sheet, the cost for the plastic sheet can be saved, and even if the polyethylene sheet is burnt, it is simply melted without generating smoke, and the products to be generated by the combustion thereof are limited to water and carbon dioxide, giving no harm to lives.
When the polyethylene sheet having a density ranging from 0.910 to 0.925 g/cm3 is employed, it is possible to obtain excellent properties in terms of transparency, formability, heat adhesiveness, extensibility and impact strength.
Further, when the polyethylene sheet having a thickness ranging from 0.07 to 0.30 mm is employed, it is possible to obtain excellent results in terms of strength, waterproofness and adhesivity to the back surface of the cement substrate.
On the other hand, a feature of the building board manufacturing apparatus proposed by the present invention is in that it is provided with a backing means to thermally adhere the plastic sheet to the ground of cement substrate, thus backing the cement substrate.
When the backing means is designed to perform the backing of a building board in subsequent to a clear coating, it is possible to utilize the heated temperature of the building board being transferred, thus dispensing after that with a high-temperature heating step for the plastic sheet, thereby making it possible to prevent the plastic sheet from being damaged in the heating step.
A feature of the building board proposed by the present invention is in that a ground of cement substrate is backed with a foamed plastic sheet.
When the foamed plastic sheet is formed, as a main component, of polyethylene, it is possible to provide a building board which is especially excellent in low temperature properties, flexibility and toughness.
Further, when the foamed plastic sheet having a thickness ranging from 0.5 to 3.0 mm is formed, it is possible to provide the plastic sheet which is excellent in strength and shape.
Furthermore, a feature of the building board manufacturing apparatus proposed by the present invention is in that it is provided with a backing means to perform the backing of the ground of cement substrate with a foamed plastic during the foam molding of an expandable plastic, thereby making it possible to obtain an excellent adhesion property of the building board.
When the backing means is designed to perform the backing of a building board in subsequent to a clear coating, it is possible to utilize the heated temperature of the building board being transferred for the foaming treatment of the expandable plastic, thus dispensing after that with a high-temperature heating step for the foamed plastic, thereby making it possible to prevent the foamed plastic from being damaged in the heating step.
There is also provided by the present invention a prefoamed plastics comprising a plastic sheet on which un-foamed plastics shaped like an island having approximately the same configuration and size as those of a building board is laminated at intervals and sequentially. When the prefoamed plastics is formed in this manner, the prefoamed plastics to be employed for backing a building board can be effectively utilized without being wasted.
Further, when the plastic sheet is formed, as a main component, of PET (polyethylene terephthalate), and the un-foamed plastics is formed, as a main component, of polyethylene, it is possible to provide a building board which is harmless and especially excellent in low temperature properties, flexibility and toughness.
The building board according to the present invention comprises a ground of cement substrate which is backed with a foamed plastic layer and a non-foamed plastic layer, the non-foamed plastic layer being disposed to constitute an outermost layer.
When the foamed plastic layer is formed, as a main component, of polyethylene, it is possible to provide a building board which is especially excellent in low temperature properties, flexibility and toughness.
Further, when the foamed plastic layer having a thickness ranging from 0.5 to 3.0 mm is formed, it is possible to provide the plastic sheet which is excellent in strength and shape.
Further, when the non-foamed plastic layer is formed, as a main component, of PET (polyethylene terephthalate), it is possible to provide a building board which is harmless and especially excellent in low temperature properties, flexibility and toughness.
Additionally, another feature of the building board manufacturing apparatus proposed by the present invention is that it is provided with a backing means which is designed to laminate a foamed plastic layer and a non-foamed plastic layer on a ground of cement substrate so as to back the cement substrate in such a manner that the non-foamed plastic layer becomes an outermost layer at the step of foam-molding the expandable plastic layer on the non-foamed plastic layer.
When the backing means is designed to perform the backing of a building board in subsequent to a clear coating, it is possible to utilize the heated temperature of the building board being transferred for the foaming treatment of the expandable plastic, thus dispensing with a high-temperature heating step for the foamed plastic, thereby making it possible to prevent the plastic layer from being damaged in the heating step.